The present invention relates generally to methods for obtaining substantially uniform profiles or shapes of the lines of deflection of the roll mantles of an adjustable or variable-crown roll and a counter-roll at a nip formed between them, wherein the bearing distance of the counter-roll is substantially larger than the corresponding bearing distance of the adjustable-crown roll, and wherein the load in the nip between the rolls is produced by loading forces applied to one or both of the adjustable-crown roll and the counter-roll, such as at regions of the bearings thereof.
The present invention also relates generally to variable-crown rolls including a fixed, central axle and a roll mantle rotatably mounted on the axle defining a space therebetween in which apparatus for adjusting the nip profile formed by the roll with a counter-roll can be adjusted. Such nip profile adjustment apparatus can include hydrodynamic glide-shoes mounted on pressure fluid controlled loading-pistons or one or more pressure-fluid chambers occupying the space between the central axle and the inner surface of the roll mantle. The glide-shoe piston devices or the like are arranged to act substantially in the plane of the nip or symmetrically with respect to the nip. The roll mantle is rotatably journalled on the central axle at regions proximate to both of its ends by means of bearings while the fixed central axle is typically supported at locations axially outside of the locations at which the roll mantle is journalled on the central fixed shaft.
Variable-crown rolls of the type described above are commonly used in paper machines to form dewatering press nips, smoothing nips, calendering nips and the like with counter-rolls. For such purposes, it is important that the distribution of the linear load, i.e., the profile of the nip in the axial direction of the rolls, is either constant or adjustable as desired, such, for example, so as to control the transverse moisture profile and/or thickness profile or caliper of the web. There are many examples in the prior art of variable or adjustable-crown rolls which are designed in an attempt to provide adjustability for the distribution of the linear load in a nip formed by such rolls and corresponding counter-rolls.
Conventional adjustable- or variable-crown rolls used in paper machines generally comprise either a solid or tubular, fixed central roll axle or shaft and a roll mantle rotatably mounted around the fixed axle. Hydraulic pressure-actuated glide-shoe arrangements and/or one or a series of pressure-fluid chambers are arranged within the space between the fixed axle and the roll mantle for aligning or adjusting the axial profile of the mantle at the nip. Generally, the nips formed by such variable-crown rolls, such as press nips or calendering nips, are loaded by means of forces applied to the axle journals of the variable-crown roll and/or the associated counter-roll.
Reference is made to FI patent application Ser. No. 822,393, corresponding to U.S. Pat. No. 4,414,890 FI published patent applications Nos. 67,923 and 69,913 corresponding to U.S. Pat. Nos. 4,327,468 and 4,440,077 respectively, and U.S. Pat. No. 3,097,590 as representative of relevant prior art.
The counter-roll forming the nip with the variable-crown roll generally is rotatably mounted on axle journals which are permanently fixed to the mantle of the counter-roll. On the other hand, the variable-crown roll is journalled on the fixed, central axle, which is generally quite massive. The distance between the opposite journal bearings of the counter-roll usually differs from the corresponding distance between the journal bearings of the variable-crown roll, the latter usually being substantially smaller than the former, thereby resulting in certain drawbacks discussed in greater detail with reference to FIG. A.
For example, the difference between the bearing distances of the variable-crown roll and its counter-roll can result in the shapes of the crowns of the variable-crown roll and its counter-roll differing from each other. Various attempts have been made to resolve this problem. For example the rolls can be crowned. However, this technique will result in a uniform nip profile at only one particular linear load. Heating devices have been used in calender rolls to heat the roll mantle at portions at which higher linear loads are desired. However, such techniques also have their own limitations and drawbacks. Attempts have also been made to obtain a uniform nip profile by designing the variable-crown roll to have an extended bearing distance. However, this technique only results in creating new problems in connection with the frame construction of the paper machine. It has also been suggested to use an articulated bearing and a rolling bearing, situated one over the other, in the journalling of a variable-crown roll and its counter-roll to obtain a uniform nip profile. However, this technique requires a significant amount of space. Still other techniques for obtaining a uniform nip profile have been suggested, such as so-called self-acting counter-moment rolls.
It has also been suggested to provide a variable-crown roll with an extended mantle so that the bearing distances of the variable-crown roll and counter-roll are substantially equal (see, e.g. FIG. B). In such arrangements, the crown line of the variable-crown roll will substantially correspond to the crown line of the counter-roll whereby the caliper profile of the nip formed by the variable-crown roll and the counter-roll is substantially uniform, as discussed in greater detail with reference to FIGS. B and D. Extending the mantle of the variable-crown roll, however, results in numerous drawbacks.